X-ray diffraction A technique used to determine crystal planar spacing in minerals.

xenobiotic A compound foreign to biological systems. Often refers to human-made compounds that are resistant or recalcitrant to biodegradation and/or decomposition.

Xeralfs [soil taxonomy] A suborder of Alfisols with Meditereanean climates (cool, moist winters and warm, dry summers) They are brownish or reddish throughout. Xeralfs have a thermic, mesic, or frigid soil temperature regime. Xeralfs have a xeric soil moisture regime and are dry for extended periods in summer (USDA, 1999; Appendix 1).

Xerands [soil taxonomy] A suborder of Andisols with Meditereanean climates (cool, moist winters and warm, dry summers) that are more or less well drained. Characteristically, Xerands have an ochric or mollic epipedon and a cambic horizon. Most Xerands have a frigid, mesic temperature regime, but may have a, thermic regime. Xerands have a xeric soil moisture regime and are dry for extended periods in summer (USDA, 1999; Appendix 1).

Xerepts [soil taxonomy] A suborder of Inceptisols with Meditereanean climates (cool, moist winters and warm, dry summers) that are more or less freely drained. Commonly, Xerepts have an ochric epipedon and a cambic horizon; some have an umbric epipedon or a duripan, some have a fragipan, and some are calcareous at a shallow depth and have a Bk or calcic horizon. Xerepts have a frigid, mesic, or thermic temperature regime. They have a xeric soil moisture regime and are dry for extended periods in summer (USDA, 1999; Appendix I).

Xererts [soil taxonomy] A suborder of Vertisols with Meditereanean climates (cool, moist winters and warm, dry summers) that are more or less freely drained. They have cracks that regularly open and close each year. Xererts may have a duripan, calcic or petrocalcic horizons. Xerets have a frigid, mesic, or thermic soil temperature regime and have a xeric soil moisture regime and are dry for extended periods in summer. USDA, 1999;Appendix 1

xeric soil moisture regime [soil taxonomy] A soil moisture regime common to mediterranean climates that have cool moist winters and warm dry summers. A limited amount of water is present but does not occur at optimum periods for plant growth. Irrigation or summer-fallow is commonly necessary for crop production.

Xerolls [soil taxonomy] A suborder of Mollisols with Meditereanean climates (cool, moist winters and warm, dry summers) that are more or less freely drained. Characteristically, they have a relatively thick mollic epipedon, a cambic or argillic horizon, and an accumulation of carbonates in the lower part of the B horizon. Xerolls have a frigid, or warmer soil temperature regime. Xerolls generally that have a xeric soil moisture regime, but some Xerolls are marginal to Aridisols and have an aridic moisture regime. They are dry for extended periods in summer (USDA, 1999; Appendix 1).

Xerults [soil taxonomy] A suborder of Ultisols with Meditereanean climates (cool, moist winters and warm, dry summers), that are more or less freely drained, and have low or moderate organic carbon content. Commonly they, are brownish or reddish throughout, and have an ochric or umbric epipedon that rests on a brownish to reddish argillic or kandic horizon. Xerults have a xeric soil moisture regime and are dry for extended periods in summer (USDA, 1999; Appendix 1).